1. Field of the Invention
The present invention relates to an alcohol acetyltransferase ("AATase") produced by, for example, Saccharomyces cerevisiae, a DNA sequence encoding, i.e., having an ability for biotechnologically producing, AATase, and a yeast having an improved ester producing ability due to the transformation with the DNA sequence. The present invention also relates to a process for producing an alcoholic beverage having an enhanced ester flavor.
2. Related Art
It is well known that acetate esters affect the flavor quality of alcoholic beverages such as sake, beer, wine and whisky. These esters are in general present in the fermented supernatant, because yeast produces a various kinds of alcohols which are further converted into esters during a fermentation procedure.
In particular, isoamyl acetate is an ester which provides a good fruity flavor for alcoholic beverages. It has been suggested that the ratio of isomyl acetate to isoamyl alcohol, which is a precursor of isomyl acetate, is closely related to the evaluation value of the sensary test. For example, sake having a great ratio of isomyl acetate to isoamyl alcohol valued as "Ginjo-shu" in the sensary test (JOHSHI HOKOKU, No. 145, P. 26 (1973).
As previously reported by Yoshioka et al., Agric. Biol. Chem., 45, 2188 (1981), AATase is an enzyme which plays primary role in the production of isoamyl acetate. The AATase synthesizes isoamyl acetate by the condensation of isoamyl alcohol and acetyl-CoA. Furthermore, AATase has been known to have a wide substrate specificity and to produce many acetate esters such as ethyl acetate in the same mechanism as described above.
Therefore, in order to increase the esters, such as isoamyl acetate in the alcoholic beverages, it is effective to enhance the AATase activity of a yeast. Some of the conventional consideration in the production of the alcoholic beverages, for example, selecting raw materials or controlling fermentation conditions, as a result, have enhanced the activity of the AATase.
However, though it has been well known that AATase is important enzyme for the production of esters, there are few reports referring. to the AATase. Partial purifications of the enzyme have been described in some reports (for example, NIPPON NOGEI KAGAKUKAISHI, 63, 435 (1989); Agric. Biol. Chem., 54, 1485 (1990); NIPPON JOZO KYOKAISHI, 87, 334 (1992), but, because AATase has very labile activity, complete purification of AATase, and the cloning of the gene encoding AATase has not been reported, so far.